The present invention relates to fluid piping systems and more particularly to piping systems of the type wherein a metallic housing is lined with a non-metallic material selected from the group consisting of elastomers and plastomers.
Lined piping systems of the foregoing type that are presently in use are generally fabricated from Schedule 40 or Schedule 80 carbon steel and lined with various chemically resistant plastics having nominal wall thicknesses usually ranging from about 0.054" for 1/2" pipe to about 0.185" or greater for a 12" pipe for polytetrafluoroethylene (PTFE) resin, and thicker for other resins such as polypropylene. Occasionally, Schedule 10 carbon steel has been used. Generally, connections are made between sections of such pipe by flange joints, in which case the lining is brought out of the pipe and flared over the face of the flange both to protect the metal from the fluids being carried and to serve as a gasket. The flanges have been predominantly of the bolted kind, and at one time it was considered desirable to include a fibrous backup washer around the liner between the metal flange face and the overlying plastic flare to compensate for cold flow of the plastic.
The known piping is quite heavy, and bolting up standard flanges requires considerable man hours to effect a typical installation. However, there has been a general reluctance in the market place to trust joints other than bolted flanges. Nevertheless, there is a need, particularly in the light of rising costs of capital improvements, for a lighter weight, more quickly assembleable, plastic lined piping system.
An early attempt in this direction is represented by the pipe assembly disclosed in U.S. Pat. No. 4,313,625 issued Feb. 2, 1982. The abstract of said patent refers to a lined pipe assembly wherein the lined pipe and liner are deformed to provide an outwardly flaring flange, a loose tapered ring is disposed on the pipe adjacent the flange and the pipe is joined to a similar pipe or cast fitting by means of a split "V" clamp. However, loose rings are the bane of the workman. If they are made sufficiently snug to restrict travel along the pipe there is the risk that they will hang up on the pipe when the joint is assembled preventing the development of a uniformly sealed joint. Therefore, a certain looseness is required and it becomes a problem to hold the rings in place until the "V" clamp can be installed.
A significant improvement over said patented pipe assembly was the basis of a copending patent application, Ser. No. 496,883, filed May 23, 1983, now U.S. Pat. No. 4,494,776, and assigned to the same assignee as the present application. In accordance with the invention described in said copending application a seal load ring having a radial face and a tapered face is installed between the flared gasket portion of the piping plastic liner and frustoconically flared flange end of a section of light-weight (e.g., Schedule 10) metal piping. The radial face of the ring is provided with a plurality of concentric circular grooves to resist plastic cold flow, and the connection is effected by a split V-clamp. The outside diameter of the load ring substantially matches that of the flared flange on the piping.
The assembly contemplated by said application necessarily limited flaring of the pipe flange to something less than 90.degree. . The reason for this limitation is that any attempt to flare light-weight or Schedule 10 piping to the standard lap diameter at a full 90.degree. angle tends to result in an imperfect lap, cracks developing, and the material thinning excessively towards the radially outer boundary. Such problems, albeit to a lesser degree, are encountered even with a conical flare as the angle increases much above about 45.degree..
With plastic lined pipe there is a certain minimum requirement for the radial dimension of the gasket seal portion of the liner that is flared over a flange face or the like. Even when the flared portion is trapped in a series of annular grooves formed in the metal that backs up the flare, cold flow is of such significance that the flare must have at least some minimum radial dimension between its inside and outside diameters for each pipe size or leakage will develop in the connection. Fluid sealing between plastic gaskets, as with any gasket material, is a function of the unit pressure exerted by one member against the other. A certain minimum sealing pressure is required to contain a given level of fluid pressure. But the higher the sealing pressure the greater the tendency to cold flow. Consequently, for a given fluid pressure rating a certain minimum radial flare dimension is required in order to maintain the necessary sealing pressure within tolerable limits. These dimensions are specified in various industrial standards. For polytetrafluorothylene resin the minimum PTFE flare diameters are specified in table 2 (copied below) of the standard ASTM F423-82 entitled "Polytetrafluoroethylene (PTFE) Plastic-Lined Ferrous Metal Pipe, Fittings, and Flanges."
TABLE 2 ______________________________________ PTFE Flare Diameter Nominal Pipe Size, Minimum PTFE Flare Diameter, in. in. (mm) ______________________________________ 1/2 11/4 (31.8) 3/4 1 9/16 (39.7) 1 17/8 (47.6) 11/2 2 11/16 (68.3) 2 3 7/16 (87.3) 3 45/8 (117.5) 4 5 15/16 (150.8) 6 8 (203.2) 8 10 1/16 (255.6) 10 121/4 (311.2) 12 143/8 (365.1) 14 151/2 (393.7) 16 173/4 (450.9) 18 201/2 (514.4) 20 221/4 (565.2) 24 261/4 (666.8) ______________________________________
Applying standard linear regression equations to the above data provides a straight line fit defined by the equation Y=0.9500134x-1.181943 where x is the minimum flare diameter in inches and y is the nominal pipe size in inches.
With prior joint assemblies the lap formed on the end of the pipe had to equal said minimum flare dimension. Unfortunately, it was then necessary to use a pipe having greater ductility and elongation capability in order to produce the required lap. Such pipe is generally more costly.
An object of the present invention is to provide a lined piping assembly which enables use of less expensive pipe grades in varying schedules of wall thickness without sacrifice in fluid handling performance.